The increasing demand of materials being used under high temperature conditions such as in gas turbine construction requires the materials to be tested very carefully at high temperatures or under similar high temperature conditions. Presently, maximum temperatures of 950.degree. C. are usable with clamping units composed of metal and being used in an oxidizing atmosphere. The dimensional stability and resistance of the clamping mechanism cannot be ensured when metal is being used because the metal is no longer resistant to oxidation at temperatures higher than 950.degree. C. Consequently, deformations may occur as noted and furthermore the specimen or workpiece may not be easily removed from the clamping mechanism after the assembly has been exposed to the higher temperatures.